| The development of electrocatalysts with dual functions and high efficiency for water splitting is a hot research topic in the field of energy.Considering the different reactants involved in the cathodic hydrogen evolution reaction(HER)and anodic oxygen evolution reaction(OER)with the increase of reaction barrier caused by complex electron transfer process,the construction of material and regulation of interface become the key factors to improve the efficiency of water splitting.Graphitic carbon nitride(g-C3N4)material,as the most common carrier material,integrates the necessary intrinsic activity,adjustability and stability in the reaction process of electrocatalyst.Therefore,taking g-C3N4 as the design substrate,the functional application of composite catalyst can be effectively realized.Based on this,the thesis takes micro-nano structure design as the starting point,combining the in-situ oxygen defect regulation mechanism,the heterogeneous interface "electronic complementary" regulation mechanism and the three-phase system bidirectional regulation mechanism.By selecting different kinds of composite phase,the electrocatalysts with transferable active center are constructed on g-C3N4 substrate.In addition,a smart self-regulation strategy is proposed,in which the amphotic conjugate ligand is selected as the intelligent induction "antenna" to construct a pH stimulus-responsive smart bifunctional gC3N4-based electrocatalyst.On this basis,the relationship between structure and performance is established,and the effect of interface regulation on performance is systematically studied,as well as the universal application rule is obtained.The main research contents are as follows:(1)Based on the high reaction barrier required by the multi-electron transfer of OER process involved in the water splitting reaction,the improvement scheme of OER performance is explored.Co-C3N4/CoOOH,which is rich in oxygen defects,is obtained by hydrothermal and further oil bath oxidation using Co-C3N4(Co-C3N4)as the substrate after pretreatment in acidic environment.The characterization and test results show that the generation of Co3+ caused by abundant oxygen defects acts as the active site of OH-adsorption,which could effectively regulate the interaction of surface-oxygen-containing species and promote the adsorption of reactants,thus improving the OER performance.CoC3N4/CoOOH electrocatalyst can realize efficient OER process in alkaline environment,with an overpotential of 170 mV at 10 mA cm-2.And the stability is also more outstanding due to the assistance of Co-C3N4.(2)Due to the adverse effects of HER process under acidic environment,the green cleaning characteristics of the process are weakened.The design of excellent acidic and alkaline universal HER electrocatalyst plays an important role in promoting the development of the energy field.In this section,homogeneous and dispersed MoS2 nano sheets are grown in situ on the base of sulfur and molybdenum co-doped g-C3N4(Mo-S-C3N4),and the active sites at the interface are regulated,so as to obtain a strongly coupled MoS2@Mo-S-C3N4 heterostructural electrocatalyst with transferable active sites for HER in acidic and alkaline media.The experimental comparison and theoretical deduction show that the unsaturated S-site on the edge of MoS2 and the active N atom on the Mo-SC3N4 base are the active centers of acidic and basic HER,respectively.Specifically,Mo-S-C3N4 is the active species of acidic HER,and MoS2 is the auxiliary catalyst.In the alkaline environment,the active site is transferred to Mo-S-C3N4,MoS2 acts as the auxiliary catalyst,and the coordination of electrons between the interface achieves a collaborative balance,which provides the best position for the adsorption of reactants.MoS2@Mo-S-C3N4 electrocatalysts exhibit overpotential of 193 and 290 mV in 0.5 M H2SO4 and 1 M KOH,respectively.(3)Maximizing the available space of the electrocatalyst and fine-tuning the interface geometry and electronic structure to promote HER and OER efficiency has been the focus of research.Based on the improvement of bifunctional HER by the two heterogeneous structures mentioned above,a construction strategy of uniform porous nanoparticle composite phase is proposed.The heterogeneous structure of Mo2N/Co-C3N4 nanoparticles with pore size of 1.13 nm is obtained by molybdenum nitride(Mo2N)combining Co anchored g-C3N4.Exploring the variation of charge distribution at the interface based on the principle of "electron complementarity" shows that the affinity between the active site Co and oxygencontaining species in OER process,as well as the adsorption and cleavage capacity of HER reactants at the active site are effectively optimized under the regulation of nitrogen species with high electronegativity.Thus,Mo2N/Co-C3N4 not only inherits the functionality of each component,but also provides an ideal heterogeneous interface.It shows impressive bifunctional activity in the electrocatalytic process,providing 10 mA cm-2 current density for HER and OER with only 100 and 210 mV,respectively.In addition,Mo2N/Co-C3N4 catalysts also show high overall water splitting stability and slight decrease in current density after 95 h.(4)Direct combination of different active components will lead to the inevitable competition in the process of reaction.In order to maximize the prevention of this competition relationship,an efficient three-phase bidirectional regulation bifunctional electrocatalyst is developed through the construction method of bifunctional groups.The homogeneous silica(SiO2)nanospheres prepared by Stober method are used as templates,a three-dimensional(3D)hollow spherical electrocatalyst SnO2@g-C3N4@SnS2 with the inner wall of tin dioxide(SnO2)and the outer wall of tin disulfide(SnS2)is obtained by adopting a step-bystep self-assembly strategy,realizing the dual functions of different interfaces controlled by g-C3N4.Benefiting from this design,the inner wall SnO2@g-C3N4 plays a major role in OER,while the outer wall g-C3N4@SnS2 contributes more to HER.Specifically,the adjustment of element C enhances the adsorption of*H at the edge S site,and the adjustment of element N optimizes the adsorption of OER reactants and intermediates,thus improving the bifunctional electrocatalytic activity.The results show that the electrocatalytic properties of SnO2@gC3N4@SnS2 in alkaline medium are enhanced in different amplitude,and 48 h stabilit is also demonstrated in the overall water splitting test.(5)The influence of electrical conductivity on the performance of electrocatalyst cannot be ignored.Based on the preparation of three-phase bidirectional regulation catalyst,Mo2C nanowires with good conductivity are prepared by precursor pyrolysis method as a fixed template.The Mo2C@gC3N4@NiMn-LDH electrocatalyst with Mo2C and NiMn-LDH distribution on both sides of g-C3N4 is obtained by self-assembly strategy.The composites exhibit excellent bifunctional electrocatalytic performance,in which Mo2C@g-C3N4 is the main catalytic active center of HER,and external NiMn-LDH acts as cocatalyst.In the OER process,the active center is transferred to g-C3N4@NiMnLDH and the internal Mo2C acts as cocatalyst.Mo2C@g-C3N4@NiMn-LDH electrocatalyst has an overpotential of 116 mV(alkaline HER)and 290 mV(alkaline OER).In addition,the potential required for water splitting is only 1.587 V,and the sample has excellent stability under constant applied voltage.(6)Since it is difficult for a single catalyst to meet the requirements of surface structure characteristics of alkaline OER and acidic HER processes at the same time,the construction of pH-responsive smart materials provides a new opportunity to solve the problem that traditional electrocatalysts cannot achieve the activity of alkaline OER and acidic HER simultaneously.In this chapter,a 3dblock metal-anchored graphite carbon nitride(3d metal-C3N4)smart electrocatalysts modified with amphotatically conjugated ligands(BDC-NH2)are constructed,and self-adaptation of electronic structure is realized by self-response of pH-stimulation,thus realizing the self-adjustment of alkaline OER and acidic HER.Specifically,the amino and carboxyl functional groups in BDC-NH2 undergo protonation and deprotonation respectively under different pH stimulation for adapting to environmental changes.Meanwhile,combined with the calculation of DFT,the increase or decrease of electron delocalization range brought by the self-response characteristics will lead to the redistribution of Bader charge around the modified active sites.As an excellent example,the OER and HER activities of BDC-NH2@Co-C3N4 are greatly increased by about 4.8 and 8.5 times after the self-adaptive process induced by BDC-NH2 in different environments.The calculations found that the improved performance is attributable to the decreased energy barriers from O*to OOH*and ΔGH*.Impressively,the proposed BDC-NH2-induced smart modulation strategy is appled to a series of 3d metal-C3N4,including Co,Ni,and Fe,providing a general structural upgrading scheme for constructing smart electrocatalytic systems. |
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